1. Field of the Invention
This invention relates to the thermal cracking (pyrolysis) of hydrocarbonaceous materials to form a plurality of individual chemical products. More particularly, this invention relates to the expansion of the product slate of individual chemicals produced by a conventional pyrolysis plant.
2. Description of the Prior Art
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes. In such an olefin production plant (cracking plant, pyrolysis plant, or plant), a hydrocarbonaceous feedstock such as ethane, naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. This mixture, after preheating, is subjected to hydrocarbon thermal cracking at elevated temperatures (1,450 to 1,550 degrees Fahrenheit or F.) in a pyrolysis furnace (steam cracker or cracker).
The cracked product effluent of the pyrolysis furnace (furnace) contains hot, gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule, or C1 to C35 inclusive, both saturated and unsaturated). This product contains aliphatics (alkanes and alkenes), alicyclics (cyclanes, cyclenes, and cyclodienes), aromatics, and molecular hydrogen (hydrogen).
This furnace product is then subjected to further processing to produce, as products of the plant, various, separate and individual chemical product streams such as hydrogen, ethylene, propylene, fuel oil, and pyrolysis gasoline. After the separation of these individual product streams from the process, the remaining cracked product contains essentially C4 and C5 hydrocarbons, and heavier gasoline components. This remainder is fed to a debutanizer wherein a crude C4 stream is separated as overhead while the C5 and heavier stream is removed as a bottoms product.
Such a C4 stream can contain varying amounts of n-butane, isobutane, 1-butene, 2-butenes (both cis and trans isomers), isobutylene, acetylenes, and diolefins such as butadiene (both cis and trans isomers).
Thus, a cracking plant is composed of two basic sections. The first section is a thermal cracking unit that employs at least one furnace fired by at least one combustion fuel (fuel) to form the cracked gas furnace product. The second section is a separation unit that, by various fractionation processes, separates various individual product streams aforesaid from the cracked gas of the first section. These individual product streams are the final products of the plant, and are exported from the plant for marketing to third parties or used internally within the plant complex to make other products.
The thermal cracking section normally burns a mixture of combustible fuels in the heating of the cracking furnaces. Basic fuels for such furnaces are natural gas and recycled fuel gas that was produced in the plant itself.
Fuel gas is a by-product of the cracking process that is carried out in the thermal cracking section and is primarily (major amount or greater than half) a mixture of hydrogen and methane.
The individual product separation section, while making the desired individual product stream separations, routinely additionally separates at least one fuel gas stream that is suitable for combustion in a plant furnace.
Heretofore, a plant that employed natural gas as a substantial part of the fuel for its furnaces recycled essentially all of its fuel gas stream(s) to its or other plant furnaces so as to minimize the amount of natural gas that had to be purchased in order to fire the furnaces to the desired extent. This recycled fuel gas was not processed, e.g., to make same acceptable to a common carrier pipeline, for purposes of marketing same to a third party as an individual plant product as was ethylene, propylene, and the like.
Synthesis gas (syngas) is made by way of several basic and well known processes, including the reforming process and the partial oxidation process, otherwise known as gasification. The steam reforming process reacts hydrocarbons with steam in the presence of a nickel catalyst to produce an equilibrium mixture of carbon monoxide and hydrogen. At the same time, the water gas shift reaction reacts carbon monoxide with water to produce carbon dioxide and hydrogen. The final product is thus a mixture of carbon monoxide, carbon dioxide, and hydrogen with trace amounts of methane. The hydrocarbon feed for the steam reforming process is usually natural gas, but can include hydrocarbon feeds as heavy as naphtha. For natural gas feedstocks, the hydrogen/carbon oxide ratio is typically 3.5 to 1.
The partial oxidation process reacts carbon with oxygen and steam, in a reducing atmosphere to produce a mixture of carbon monoxide, carbon dioxide, and hydrogen. Depending on the carbon source feed used in the reforming reaction and the specific processing scheme, the hydrogen/carbon oxide (H2/COx) ratio in the syngas will vary widely depending on the ratio of oxygen-to-carbon and the ratio of water-to-carbon in the feed to the reactor. Other factors include the ratio of hydrogen-to-carbon in the carbonaceous feedstock as well as operating pressure and temperatures. Feedstocks can range from methane to petroleum coke or coal to naturally occurring hydrocarbonaceous materials or waste products. This partial oxidation process is also referred to as gasification, or more specifically, as coal gasification when coal is the feed.
Syngas is combustible. Currently syngas is combusted or otherwise burned only in Integrated Gasification Combined Cycle (IGCC) plants. Syngas cannot be substituted for natural gas, e.g., in conventional common carrier natural gas pipelines, because of its high hydrogen and carbon monoxide content and consequent low heating value on a volumetric basis, Btu/cubic foot of gas. Syngas is also employed to produce chemicals as explained later, but these processes do not in any way involve the combustion of syngas.
Although IGCC plants can employ as their primary feedstock a number of hydrocarbonaceous materials such as coal, oil, coke, refinery bottoms, biomass, and certain waste materials (municipal, hazardous, etc.), they find their roots in the evolution of coal gasification. The IGCC description will, for sake of clarity, hereafter be directed toward coal, but this is not to exclude the other feedstocks just mentioned.
Coal gasification to produce a commercial fuel is approaching 200 years of age in the United States, the first such application starting in 1816 in Baltimore to provide lighting in that city. By 1920 coal gas served an estimated 46 million people. Coal gas use declined in the 1930s and 1940s with the increasing availability of natural gas from Texas and Louisiana, but in the 1930s and 1940s Germany developed processes for producing gasoline, diesel, and other liquid fuels from coal. In this regard, the Fisher-Tropsch (F-T) process was developed and is still employed with syngas at present. F-T is a catalytic reaction that produces longer chain hydrocarbons (synfuels) from syngas.
Interest in the production of synfuels was stimulated by the energy crises of the 1970s which ultimately led to the IGCC process. This process involves an endothermic chemical conversion (partial oxidation) of a feed such as coal into syngas. The conversion is carried out in a gasifier that employs a maximum amount of carbon, a high temperature, a minimum amount of oxygen, and water. The raw syngas formed in the gasifier is then cleaned by removing particulates and chemical contaminates such as hydrogen sulfide, carbonyl sulfide, ammonia, and chlorides. The cleaned syngas is fed to a combustion turbine that drives an electric generator to produce electricity to feed into the power grid.
Hot exhaust gas from the combustion turbine generator plus process heat from the gasifier itself is passed to a waste heat recovery steam generator which drives a steam turbine/electric generator to produce additional electricity for the power grid. The combination of the combustion turbine generator and steam turbine generator together with intermediate heat recovery and steam generation is referred to as “combined cycle” and is the “CC” in IGCC.
Thus, IGCC technology is the integration of carbon gasification with combined cycle, and this combination significantly improves the efficiency for utilizing hydrocarbonaceous feeds as set forth hereinabove for electrical generation purposes with concomitant low pollutant formation.
IGCC technology is now proven and well established. It has been demonstrated with coal at a commercial scale for up to ten years at two sites in the United States and two in Europe. Although these IGCC plants were originally demonstration plants, they are now in regular commercial operation.
As already mentioned, cleaned syngas from an IGCC plant can be combusted in a gas turbine context. Alternatively, syngas can be employed in the production of chemicals such as hydrogen, carbon monoxide, fertilizer, methanol, ethanol, and other industrial chemicals; or in F-T processing to produce naphtha, diesel fuel, jet fuel, and wax; or to produce synthetic natural gas.
However, none of these alternative uses for syngas involve the combustion of the syngas, IGCC plants being the one and only technology at present that can employ syngas as a combustion fuel.
Syngas typically has an H2/CO molar ratio of about 0.4/1 to about 0.7/1. It has a heating value of only about 260 to about 280 Btu per standard cubic foot (Btu/SCF) as compared to about 950 to about 1,100 Btu/SCF for natural gas. Syngas does not, therefore, come even close to the Btu value specification for common carrier natural gas pipelines. Accordingly, syngas is not a simple substitute as a combustion fuel, especially natural gas. For example, in a natural gas fired turbine, the fuel gas is only about two percent of the total gas flow and the remainder is air for dilution and combustion purposes. On the other hand, if syngas and the required diluent were to be substituted for natural gas in this application, it would account for fourteen to sixteen percent of the total gas flow, a very considerable increase in mass flow that turbine operators must seriously take into account. Another example is dry low NOx combustors. These combustors cannot use syngas as a combustion fuel because of its high hydrogen content which gives syngas a high flame speed that can initiate flashback and cause combustor failure. Syngas can also adversely affect the heat flux distribution between the radiant and convection sections of a furnace.
Accordingly, it would be desirable to find other uses for syngas as a combustion fuel. This invention does just that in the thermal cracking area, and does so with surprising additional results.